This invention relates to the field of borehole logging. More particularly, this invention relates to a process for evaluating formation invasion and formation permeability through resistivity logging.
The principal objectives of well logging are to identify and correlate underground formations, to determine the minerology and physical properties of potential reservoir rock and the nature of the fluids they contain. Porosity and fluid saturation are of most interest in the estimation of hydrocarbon reserves. Permeability, which is a measure of the resistance of the formation to fluid flow, including invasion, and which is needed to estimate recoverable reserves, has been the most difficult parameter to determine and quantify by borehole surveys.
One of the primary difficulties in formation analysis from borehole surveys is the need to determine and compensate for the effects of invasion. Invasion takes place in porous permeable zones where the hydrostatic/dynamic pressure of the drilling mud is greater than the formation pore pressure. The invasion of the mud filtrate will cause a radial variation of the formation resistivity. Since one objective of most logging methods is to measure the properties of the undisturbed formation, logging tools are usually designed to be insensitive to invasion or compensated for this fluid replacement by means of departure curves. In the prior art, this is normally done by running wireline resistivity logs which have different response functions, or depths of investigation (i.e., spherically focused (or short-normal) logs, medium induction logs, and deep induction logs). From these measurements, invasion corrections can be derived. (See, for example, Dresser-Atlas, Log Interpretation Charts, (1979)). More recently, a different approach has been used, which is to solve the "forward" problem. In this method, one assumes a lithology, porosity and water saturation (among other parameters) and varies these until a fit is obtained to the suite of wireline logs. (See, for example, the following papers at the SPWLA Twenty-fifth Annual Loging Symposium, New Orleans, June 10-13, 1984. M. A. Yuratich and Walter J. Meger, "The Application of Finite Difference Methods to Normal Resistivity Logs," (Paper V). Mark Alberty and Khaled H. Hashmy, "Application of ULTRA to Log Analysis", (Paper Z). Yih-Yih Lin, Stan Gianzero, and Robert Strickland, "Inversion of Induction Logging Data Using the Least Squares Technique" (Paper AA). However, these wireline methods have serious drawbacks or shortcomings. In either analysis one is looking at "snapshots" of the formation at a particular time (typically many days after drilling), with each measurement having a different response function. These analyses give no information about the time dependence, i.e., the rate, of the invasion process, but are designed only to reconstruct the formation's properties prior to invasion.
These previous methods involve taking a suite of several wireline measurements days or weeks after drilling has occured at a particular depth in a borehole. Because these wireline logs are taken so long after drilling, invasion is often essentially complete and these logs provide no information about the rate of invasion, and, hence, do not provide information about permeability.
Known methods to obtain permeability information include laboratory analyses of core samples and inferences from other suites of wireline logs. These prior methods are cumbersome, time consuming and/or highly unreliable. Indeed, it is often considered to be doing well to get within an order of magnitude of the actual permeability.